Since 1965, the Soviet Union has used Molniya orbits for over eighty of her communications satellites. These orbits are inclined to the earth's equator by 63.4.degree. and are highly eccentric ellipses. They have zero apsidal rotation and very slow nodal regression, so that the apogee of the orbit can be maintained at 63.4.degree. North latitude.
In such an orbit, a satellite loiters in the apogee region at high northerly latitudes. However, as shown in FIG. 1, the satellite does not reach apogee at the same time-of-day every day of the year. Rather, the Molniya orbit's apsidal line remains oriented in approximately the same direction in inertial space, rotating westward only very slowly. Therefore, the satellite reaches apogee over the same longitude X each day, but approximately four minutes earlier each day (six hours earlier every three months). The Soviet Union overcomes this handicap by placing a constellation of sixteen Molniya satellites in orbit simultaneously, the satellites reaching the loiter zone seriatim. With the present invention, a single satellite 1 can be used to communicate with five or six preselected regions of the earth at the same fixed times each day of the year, which times may advantageously correspond to times of peak demand for satellite communications services. In the present invention, apsidal rotation is intentionally used to keep the satellite 1 over the same longitude on earth at the same time each day.
Molniya satellites usually operate in orbits with eccentricities of approximately 0.7 and periods of just under twelve hours. The instant ACE orbit has an eccentricity of 0.49 and a sidereal period of 4.79 hours, or an eccentricity of 0.23 and a sidereal period of 4 hours.
"Final Report, Voice Broadcast Mission Study, Volume II--Study Report" for NASA Contract No. NASw-1475, Jul. 14, 1967, pages 2-8 and 2-9, teaches away from the instant orbit in its rejection of elliptical orbits for communications purposes. This reference considers apsidal rotation to be a "problem".
K. M. Price, Monthly Technical Progress Narrative Report No. 1 for NASA Contract No. NAS3-24891, "The Use of Satellites in Non-Geostationary Orbits for Unloading Geostationary Communications Satellite Traffic Peaks", May 13, 1986, discloses some of the parameters of the instant ACE orbit.
U.S. Pat. No. 3,163,820 discloses orbits which satisfy one of the two following criteria: 1) the orbit has a greater radius and a longer period than for a geosynchronous orbit; and 2) the satellite has a lower angular speed in the eastward direction than that of the rotating earth. A satellite 1 in the instant ACE orbit, on the other hand, always has a higher angular speed in the eastward direction than the rotating earth. The present invention offers the following advantages over the reference's orbits: 1) a single satellite 1 in an ACE orbit can provide communications coverage for five or six preselected geographical areas of the earth for a certain duration at the same times-of-day every day of the year; and 2) an ACE orbit requires less propellant to achieve for a satellite 1 of fixed size.
U.S. Pat. No. 3,243,706 discloses a system of twelve satellites in circular orbit, as opposed to a single satellite 1 in elliptical orbit of the present invention. The present invention offers the following advantages: 1) a single satellite 1 can provide communications coverage for five or six preselected geographical areas for a certain duration at the same times-of-day every day of the year; and 2) the satellite 1 remains in view of ground terminals 2 longer, and therefore can be tracked at lower slew rates.
U.S. Pat. No. 3,349,398 discloses means for maintaining communications between a satellite and a ground terminal. Such a system could be used in conjunction with the present invention.
U.S. Pat. No. 3,706,037 discloses a system involving two satellites in near-geosynchronous orbits positioned to avoid solar outages resulting from RF interference from the sun. A ground terminal 2 tracking a satellite 1 in the instant ACE orbit slews its antenna in the same pattern each day, while there must be a day-to-day variation in the slew motion necessary to follow a satellite using the orbit described in the reference.
U.S. Pat. No. 3,836,969 discloses a means for reducing stationkeeping propellant consumption by choosing a low inclination, rather than a zero inclination, geosynchronous orbit. The present invention inherently saves propellant because less propellant is required to achieve the instant ACE orbit than a geosynchronous orbit for a fixed spacecraft 1 mass.
U.S. Pat. No. 3,852,750 shows the use of three or more satellites in geosynchronous orbit for navigational, rather than communications, purposes. The instant invention, on the other hand, is more suited for communications than navigation. Navigation requires continuous satellite coverage, which the ACE orbit does not provide unless several satellites 1 are placed in ACE orbits. Rather, the major advantage of placing a satellite 1 into an ACE orbit is to off-load daily peaks in the geostationary satellite communications traffic, peaks which tend to occur at the same time-of-day throughout the year.
U.S. Pat. No. 3,995,801 discloses the use of circular orbits at geosynchronous altitude. The present invention, on the other hand, is an elliptical equatorial orbit having a lower radius than those disclosed in the patent. By avoiding the crowded geostationary arc, antennas of satellites 1 in ACE orbits can be built smaller for the same link budget for certain telecommunications applications.
U.S. Pat. No. 4,004,098 describes a system by which two satellites can communicate between themselves. The present invention, on the other hand, is an orbit in which a single satellite 1 can provide effective communications.
U.S. Pat. No. 4,375,697 discloses multiple satellites traveling in clusters in a geostationary orbit. On the other hand, the present invention is a non-geostationary orbit in which a single satellite 1 can provide useful communications coverage.
U.S. Pat. No. 4,502,051 also discloses a satellite system having many satellites. Multiple backup satellites are required to handle on-orbit failures. This is because the satellites in the system are not in the same orbit. In the present invention, on the other hand, the communications system functions with a single satellite 1. A backup satellite 1 in the same orbit can replace it.
In the reference patent, satellites appear to cross the geostationary arc from the perspective of a ground terminal in the United States and other non-equatorial countries. A ground control station deactivates satellites at these times, to prevent interference with geostationary satellite communications. In the present invention, on the other hand, satellites 1 never appear to cross the geostationary arc from the perspective of a ground terminal 2 in the United States or other non-equatorial country. Interference with geostationary satellite communications is thus advantageously avoided.
The following three papers were published between the filing date of the parent application and that of the instant application:
K. M. Price et al., "The Use of Satellites in Non-Geostationary Orbits for Unloading Geostationary Comunications Satellite Traffic Peaks", final report for NASA Contract NAS3-24891.
A. E. Turner, "New Non-Geosynchronous Orbits for Communications Satellites to Off-Load Daily Peaks in Geostationary Traffic", handout accompanying oral presentation given at meeting of American Institute of Aeronautics and Astronautics and American Astronautical Society in Kalispell, Montana, Aug. 10, 1987.
G. H. Stevens et al., "Complementary Satellite Sound Broadcasting Systems", a NASA assessment for the Voice of America presented in Washington, D.C., April, 1987.